In Jackass Presents: Bad Grandpa, star Johnny Knoxville fractured his elbow, had to undergo surgery for a ruptured tendon in his knee, and ruptured another tendon in his hand while shooting a promotion for the film at a college frat house. For any other actor, this would be an alarming series of injuries. For Knoxville, it was, in his words, "getting away easy."

Since 2000, Knoxville has willfully allowed himself to come to enormous amount of harm in the service of the Jackass franchise, first on the MTV reality series (and subsequent spin-offs), and then in three hugely successful feature films. He has broken bones, sustained concussions, and even once broke his penis. (Click here to watch the stunt that caused that particular injury, and be prepared to cross your legs in empathetic agony.)

The injuries are an unavoidable consequence of the brilliantly simple Jackass model for physical comedy: Think of a crazy stunt that could cause all manner of bodily injury, do it, and film it. Bad Grandpa, however, introduced two new elements to the mix. The first: a bona fide storyline, involving one of Knoxville's Jackass characters, Irving Zisman, taking his grandson (actor Jackson Nicoll) on a wild cross-country trip. The second: Instead of a series of relatively controlled stunts, Knoxville interacted with actual people captured by hidden cameras, more often than not playing out not as stunts, but as far less dangerous pranks. When Knoxville did go through a real stunt — like when he crashes through a department store window — it had to be far more controlled in order to keep those real live people from harm, including a "safe zone" of 20 to 30 feet and a stunt coordinator within eyeshot to let Knoxville know when he could trigger the stunt.

As a result, said Knoxville, "I wasn't injured as much in Bad Grandpa as I was in the Jackass films."

It would be easy to assume that Knoxville, now 42, embraced this kindler, gentler style of Jackass movie because he's perhaps a little tired of visiting the ER multiple times a shoot. But it would also be wrong. "I'm not dialing things down at all," he said. In fact, when Knoxville and co-writers Spike Jonze and Jeff Tremaine (who also directed) were cooking up the film's screenplay, they had planed for Knoxville to undergo some far more elaborate stunts. "We usually save the bigger stunts until the end of the film," said Knoxville. "By the time we got around to committing to do some of those stunts, we had so much footage, and the story was working so much by that time, that we thought, well, we just won't do that."

It was a brand-new concept for Knoxville — serving the story rather than the stunts — and he found it liberating. "I think this movie blew everything wide open for us as far as possibilities," he said. "We didn't know if we could pull this off in the beginning, but we did, and we're very proud of that and reenergized by it."

But Knoxville also wanted to be clear: When it comes to undertaking hilariously dangerous stunts, "right now I have the desire, the urge. I'm as barely able as ever." But why?

It's a question that seemed to stymie Knoxville at first. "I don't spend my energy thinking about stuff that I don't have the desire for," he said. "When the time comes [to stop], it comes." He explained that doctors have told him he has a high pain threshold, a concept Knoxville finds hard to grasp since he has no point of reference for the pain threshold for mere mortals. "I think it's my 'give a shit' tolerance," he said. "I know what's gonna happen. You're about to get wrecked, and I just kind of — I'm OK with it, because I want footage. The producer side overrides the performer side. Every time. Every fucking time."

Knoxville did admit that he is "hyper-limber," which allowed him to handle a scene in Bad Grandpa where Zisman gets trapped in a malfunctioning adjustable bed with ease. "That wasn't even a stunt," he said. "That was just sitting up in bed." (You can watch a clip of the scene above.)

When I asked Knoxville if he was ever amazed at his ability to bounce back from horrible injuries, again and again, he finally did pause for a moment. "I'm not amazed by it, I just know where I'm at with my body, and I'm good to go," he said. "It's like, I grew up with asthma. Really bad asthma — I was in the hospital a couple times, almost died when I was 8. When you're little, you don't realize how sick you are because you just want to go out and do something. People are like, 'Wow, you were sick; it's amazing you could do it.' You don't even intellectualize it like that, you just want to do everything everyone is doing. With the injuries, it's like, 'Yeah, I'm injured, but I just want to keep moving.' I don't get tied down by illness or the injuries, I just want to keep moving forward."